Modern gas turbines are provided with an engine core comprising a compressor, combustor and turbine section and a surrounding annular bypass duct through which an air flow is guided by a fan. The bypass duct is limited by a radially inner wall and by a radially outer wall. Between the inner wall and the outer wall of the bypass duct, a support unit is provided that includes strut-like support elements connected at one end to the inner wall and at the other end to the outer wall.
FIG. 1 depicts a ducted fan gas turbine engine generally indicated at 10 which comprises, in axial flow series, an air intake 1, a propulsive fan 12, an intermediate pressure compressor 7, a high pressure compressor 9, combustion equipment 115, a high pressure turbine 116, an intermediate pressure turbine 117, a low pressure turbine 118 and an exhaust nozzle 119.
Air entering the air intake 8 is accelerated by the fan 12 to produce two air flows, a first air flow 11 into the intermediate pressure compressor 7 and a second air flow 10 that passes over the outer surface of the engine casing 12 and through a bypass duct 2 which provides propulsive thrust. The intermediate pressure compressor 7 compresses the air flow directed into it before delivering the air to the high pressure compressor 9 where further compression takes place.
Compressed air exhausted from the high pressure compressor 9 is directed into the combustion equipment 115, where it is mixed with fuel that is injected from a fuel injector and the mixture combusted. The resultant hot combustion products expand through and thereby drive the high 116, intermediate 117 and low pressure 118 turbines before being exhausted through the nozzle 119 to provide additional propulsive thrust. The high, intermediate and low pressure turbines respectively drive the high and intermediate pressure compressors and the fan by suitable interconnecting shafts.
In the bypass duct 2, several fan outlet guide vanes 13 are arranged downstream of the fan 9 which reduce or remove a twist in the flow of the bypass flow 10. In addition, supports 14a or 14b are provided downstream of the fan outlet guide vanes 13, bracing the inner wall 3 and the outer wall 4 against one another. In addition to the supports 14, the engine can have further support structures called bifurcations 15 through which lines are routed for supplying the jet engine device 1 or an airframe of an aircraft provided with the jet engine device 1. The position of the support 14a is the position typically used within large or medium civil gas turbines. The position of the support 14b is that typically used in smaller, business jet type applications. The general structure and form of the supports is the same for each and will be discussed in common as reference 14 in the rest of the specification.
Depending on the specific application, it is also possible for the bifurcations 15 to be arranged in the same cross-sectional plane as the supports 14.
The support unit 14 includes strut-like aerofoil support elements 17 to 20 shown in more detail in FIG. 2 and connected at one end to the inner wall 3 and at the other end to the outer wall 4. The aerofoils 17 to 20 of each support element each describe an acute angle 17E, 18E, 19E and 20E between the circumferentially outward facing flanks 17C, 18C, 19C and 20C and the radially outer wall of the bypass duct 4. The aerofoils 17 to 20 of each support element also each describe an acute angle 17F, 18F, 19F and 20F between the circumferentially inward facing flanks 17D, 18D, 19D and 20D and the radially inner wall of the bypass duct 3.
Each support unit comprises two aerofoils 17 and 18 or 19 and 20, respectively that are connected in a manner forming an A-arrangement to the inner wall 3 and to the outer wall 4 in the manner shown in more detail in FIG. 2 and form so-called A-frames of the support unit 14. The “A” tapering as the support unit progresses radially outwards. The facing flanks of each aerofoil pair describing an acute angle with the radially inner wall of the bypass duct.
The aerofoils 17 and 18 or 19 and 20, respectively, representing A-frames level in the bypass duct 2 with a duct height H in order to transmit engine loads acting in the area of the engine core 12 outwards in the direction of the outer wall 4. The aerofoils 17 and 18 or 19 and 20, respectively, assigned to one another in pairs form the support units and are, depending on the specific application, arranged relative to one another at a defined acute angle 17F, 18F or 19F, 20F, respectively, and at a distance D defined in the circumferential direction.
The acute angle of the aerofoil flanks to the radially inner and outer walls causes an increase in the velocity of the air which subsequently interacts with the main flow boundary layer and causes wakes to form which add to the pressure loss through the bypass duct and can take energy from the bulk flow. The lost energy reduces the overall efficiency of the gas turbine engine and reduces the engine performance.
In many conventional engines an offtake is provided in the by-pass duct to supply cool air for proper functioning of the engine and its units. The offtakes are separated from the aerofoils and also generate wakes as shown in FIG. 3 and therefore also generate further pressure loss in the by-pass duct.
FIG. 4 depicts the static pressure of the bypass flow 10 for the arrangement of FIG. 3. The bulk pressure has a region 30 where the pressure is relatively constant. At the leading edge of the support unit there are regions of lower static pressure 32 and similar regions of lower static pressure 34, 36 can be seen at boundary layers of the radially inner wall 3 and the radially outer wall 4 respectively.
The regions of lowest static pressure 38, 40 are found where the aerofoils of the A frame form an acute angle with the radially inner 3 and the radially outer 4 wall of the bypass duct. These regions can be significant and can cause significant wakes to form that reduce the efficiency of the gas turbine engine. The wakes are formed in part by the acute angle that the aerofoil forms to the duct wall and which causes an increase in the velocity of the air that goes on to interact with the main flow boundary layer. The wakes generate an area of increased loss in the by-pass duct flow and a reduction in engine performance.
It is an object of the present invention to seek to provide an arrangement having an improved efficiency.